Merge branch 'release/0.4' into production

* release/0.4: (32 commits)
  Bump version.
  Test against target version of Kotti that matches the requirements.txt installed before.
  Update translations.
  Adjust test.
  Get default_sender from the .ini files' mail.default_sender option if not set in the settings for the site.
  Update docs.
  Add a migration script.
  Add sender to the model and the form schema.  Use sender when sending emails.  This prevents problems w.r.t. SPF.
  Add default_sender_address to the settings.
  Fix PEP8 errors and warnings.
  Update german translations.
  Refactore widget template.
  Missing import statement.
  The captcha setting defaults to False.
  Public and private key are not required, it's possible to use the form without the captcha.
  Pin kotti to latest stable release.
  Update german translation.
  Added option to set the theme of the recaptcha widget.
  Pin wsgi_intercept in travis configuration.
  Fixing travis configuration issue.
  ...
Fix the RecaptchaWidget serializer that hasn't been working at all before.
Merge branch 'hotfix/0.4.1'

* hotfix/0.4.1:
  Fix the RecaptchaWidget serializer that hasn't been working at all before.

Conflicts:
	CHANGES.rst
	kotti_contactform/widgets.py